This invention relates generally to mass spectrometers having multiple mass analysis stages and more particularly is concerned with coupling the multiple mass analysis stages to minimize the effects of stray capacitances between the stages, especially when the stages are positioned close together.
The use of multiple quadrupole rod sets in a mass spectrometer is known. Conventionally, each quadrupole rod set has its own function. Where an individual quadrupole rod set is used as a mass analyzer, its function is often independent of the function of adjacent rod sets.
For example, U.S. Pat. No. 4,234,791 Nov. 18, 1980, xe2x80x9cTandem Quadrupole Mass Spectrometer for Selected Ion Fragmentation Studies and Low Energy Collision Induced Dissociator Thereforxe2x80x9d describes a system comprising three sets of quadrupoles in series, a configuration commonly referred to as a triple quadrupole. A first quadrupole mass analyzer selects an ion of one particular mass to charge ratio (m/e) from a mixture produced in an ion source. These selected ions then collide with a gas in a second quadrupole operated in an RF mode only. The collisions transfer translational energy to internal energy of the ions, causing the ions to fragment. A mass spectrum of the fragment ions is then obtained with a third quadrupole. The first and third quadrupoles operate with selected RF and DC voltages to give the desired mass resolution.
It has been found that a combination of several quadrupole rod sets in tandem, all operating as mass analyzers and all configured to select the same ion, can, in certain circumstances, provide a higher resolution mass analyzer. Such a configuration is disclosed in U.S. patent application Ser. No. 09/188,352. It was found preferable to position the adjacent rod sets close to one another, with no physical lens separating them. The need for this is further described below.
With the quadrupoles placed close together and with no lens between the quadrupoles it was found that capacitance coupling of the RF between the quadrupoles caused problems with the control circuits. There are many known quadrupole designs which have multiple rod sets, which are mounted close to one another. However, the problem of capacitance coupling between rod sets is not usually a problem for a number of reasons. Often one rod set is larger than another, so that the larger rod set at least will not sense any significant effect from a field from a smaller rod set. In many cases, the RF drive for one rod set is derived by a capacitance connection with another rod set or its RF driver circuit, so that adjacent rod sets are, in any event, coupled in a controlled manner. In some cases the quadrupoles operate at different frequencies so that one quadrupole power supply is not sensitive to electrical pick-up from another. Also, for many quadrupole designs, one rod set is often enclosed in a chamber, with lens at either end, so that it can be operated at a different pressure from adjacent rod sets. The lenses at either end serve not only to isolate the different pressure regions but also to provide isolation or separation between fields of the different rod sets.
Thus, in known designs, problems due to close coupling have in general not been significant. In the case of the instant device, when two quadrupole mass analyzers were positioned in close proximity, it was found that the RF field of one quadrupole power supply interfered with the second power supply due to a capacitance effect between adjacent rods.
The present invention provides a method of reducing the effects of stray capacitance between adjacent quadrupole rod sets being operated in series in mass analyzer mode to provide, in combination, a more precise mass analyzer.
In accordance with the present invention, there is provided a mass spectrometry apparatus comprising: (a) first and second multipole rod sets, each of said first and second multipole rod set having (i) two or more positive rods, all of the positive rods being coupled together and (ii) two or more negative rods, all of the negative rods being coupled together, (b) a first voltage generator coupled to the positive and negative rods of said first multipole rod set for generating a potential in the first multipole rod set, (c) a second voltage generator coupled to the positive and negative rods of said second multipole rod set for generating a potential in the second multipole rod set, (d) a first capacitor coupled between the positive rods of said first multipole rod set and the negative rods of said second multipole rod set, and (e) a second capacitor coupled between the negative rods of said first multipole rod set and the positive rods of said second multipole rod set.